This paper is a progress update on the solar thermal augmentation system for ground source heat pumps using a solarium-style polycarbonate wall-mounted ‘sunbox’ to recharge the ground, previously reported at CIBSE/ASHRAE 2012. The system is running on the author’s house in Nottingham, England. The house has a ground source heat pump, using two vertical boreholes. Real-time, diurnial and inter-seasonal solar charging processes restore the energy level in the earth, thus preventing progressive chilling of the borehole zone. This has improved the performance of the heat pump to a degree that if balanced with the 4kW PV array, it be said that the house is net-zero – generating more than it consumes, a solar fraction averaging 120% over 2.5 years.

Since CIBSE/ASHRAE 2012, there is now sufficient real data collected to construct a computer simulation, to see how it compares with the real patterns of weather and behaviour. The modelling process attempts to illustrate the effective relative energy levels from year to year and to identify the thermal elasticity of the borehole.

Since CIBSE/ASHRAE 2012, there have been technical developments of the system. One has been the addition of two additional methods of solar capture. Vacuum tubes and a new roof-mounted sunbox with metal radiators are working to the same boreholes, with independent controls, pumping and metering. The previously reported wall-mounted ‘sunbox’ has been re-fronted with double skin stretched ETFE, and the paper will include a short report about adapting ETFE for use in a solar collector. The three solar collection methods provide a way of comparing the cost effectiveness, technical and visual impact of the technology of solar augmentation of a heat pump.